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Understanding the Direct-delivery site to site
Mike sends messages to John. Both mailboxes (Mike and John) are in Exchange 2007 server.
The Hub transport server:Ex20071 makes decision based on the least cost. There are four possible routes to reach Site D. The ADLinkA-C+ADLinkC-D route is the least cost route. The Ex20071 will deliver the messages through this route.
Direct-delivery
Ex20071 directly delivers messages to Ex20072. Ex20073 has no knowledge of the message delivery. Ex20073 does not process the messages at all.
If you want the HUB Transport Server in SITE C to process the message, configure it as a hub site.
Set-AdSite -identity "Site C' -HubSiteEnabled:$true
The route selection mechanism is still the least cost path. The hub site definition only affects the direct-delivery. When Mike sends mail to John, Ex20071 delivers it to Ex20073 (Hub). Ex20073 processes it and delivers it to Ex20072.
Most of the time, you should not define a HUB Site.
Problem when the Site C is defined as the Hub Site.
Assuming the Ex20073 hub transport server is down and the Active Directory site (site C) is still working:
Ex20071 delivers messages to Ex20073 first. Because of the down status of Ex20073, Ex20071 keeps retrying the delivery.
If Site C is not defined as a HUB SITE, mail is directly delivered from SITE A to SITE D regardless Ex20073 down or up as long as Site A can physically reach the Site C.
If the AD Site C is down, the route (ADLinkA-B+ADLinkB-D) is selected for message delivery.
Why would you do that?
One example will be that a company implements the centralized journaling.
Without the hub site defined, the message delivery is using “Direct-relay”.
With the hub site defined:
Set-AdSite -identity "Site E' -HubSiteEnabled:$true
Without ExchangeCost set, Ex20071 selects ADLinkA-C+ADLinkC-D route.
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With ExchangeCost set
Set-AdSiteLink -Identity ADLinkC-D -ExchangeCost 25 -MaxMessageSize 10MB
Ex20071 will select ADLinkA-B+ADLinkB-D route for Exchange traffic. The Active Directory replication is still using the ADLinkA-C+ADLinkC-D route.
Ex20071 directly relays the message to Ex20072. Ex20074 does not process the exchange messages.
